Trying to save yourself is not only the root cause of the first sin but of all sin...

This fruit [from the tree of life] was not magical but it was a sign and a guarantee to them that God would sustain their lives in eternal bliss with Him. All they had to do was enjoy the garden the way God intended—enjoying God's goodness in the garden as it was intended. Enjoying God's goodness in the garden would have kept them from the forbidden tree [the tree of the knowledge of good and evil]... 

[Adam and Eve] stopped enjoying God and His garden and this cause them to doubt God's provision. It cause them to try to provide for themselves and seek the good, apart from God. This is where sin comes from. When we stop enjoying the grace and the gifts of God, our hearts will seek out anything else to satisfy us...

Now we as Christians have a new covenant in Christ and it is better than the old covenant... Jesus has taken that penalty of death we earned so that we can stay in covenant relationship with God... 

The surefire way to fall into sin is to forget this good news [of what Jesus has done]; to stop reveling in it and enjoying it. If we stop enjoying what God has provided in the garden of the Gospel it is certain that we will run to the forbidden tree of sin.

Conversely, the more we rejoice in all the blessings we have in Christ who kept the covenant for us, the more our hearts will be rewired to never want to take a single step towards the tree of the knowledge because of how good the fruit taste from the tree of life.

What was it that drove Eve to the tree of the knowledge of good and evil?

...If Adam and Eve in the garden were supposed to do the good God commanded and not do the bad God prohibited, then the fruit of the tree offered a unique allure: Wisdom, concerning that which was God's alone—the knowledge of good and evil.  If they knew what God knew they could do more of the good what God desired and avoid more of the evil God detested. And if doing good and avoiding evil was how the covenant was fulfilled then knowledge of what was good and evil would help Adam and Even fulfill the covenant. 

And Eve desired this knowledge, this forbidden wisdom, and her desires, her affections, got the best of her. At the very least Eve wanted to be like God, knowing good and evil. If God, being the One who knows good and evil  is the one who can either bless or curse, bring life or death, then Eve, if she were to become like God, could become the benefactor, the savior, the provider. 

This is the true nature of Eve's sin: Trying to obtain the good we desire by any other means than God's gracious giving.  

Sin is any attempt to get the good apart from God...

We convince ourselves that there is something we can do to earn our salvation or at least contribute to it... We doubt that Jesus is not really enough. We fear that His sacrifice was not really all we needed to be at peace with God. So we set out on our own, like Eve.

And this is how our hearts turn seemingly  innocent acts into grievous sins. When we depend on our own actions to save us and provide for us apart from God's grace in the Gospel, it is a sin. And this is what Eve was trying to do with the tree of the knowledge of good and evil. She was trying to find the wisdom she needed to do more for her relationship with God...

What keeps us from trying to earn our own salvation through misdirected attempts at doing good? Enjoying Jesus and His Gospel, relishing in His grace... Enjoying all God provided for us in Jesus...

Why was Even sin... trying to save herself?

..Eve was pre-breaking three of the ten commandments. 

The first commandment states, "You shall have no other gods before me." ...She placed herself  as a mini god before the One True God...  God's provision in the garden in all its fullness was not enough. She needed to provide for herself a new and better kind of food.  God's command and revealed will was insufficient to disclose all the truth necessary for her happiness, she thought. She needed to discover new revelation for herself. And ultimately, she could not put her fate and the fate of her husband in the hands of this kind of God. She needed to save herself and her family. God was not enough, so she would be her own God. 

The second commandment she  broke... "You shall not make for yourself a carved image..." This tree with this wisdom-giving fruit, had been cast and forged and enshrined within the temple of her heart. This tree would be the means for her happiness. This tree would be the god that would provided for her. This tree would be the avenue of her blessing. Eve committed idolatry. 

The third commandment Eve proactively broke is the tenth and final command.  "Do not covet..." Coveting is a special form of wanting what you don't have... It's a want that sets up as a supreme desire in your heart. It is a want that says I cannot be happy without that. It is a want that wires your heart towards sin. . Eve coveted wisdom. Eve coveted the ability to earn her own blessing. Eve coveted self-righteousness. 

Eve's chief sin in eating the fruit is... displacing God as her only source of provision, blessing, righteousness, and life. Her sin was self-preservation, self-blessedness, self-righteousness. She was a covetous, idolatrous mini-god...

{We too, trust] in our own image rather than the image of God. We now wanna save ourselves. We wanna  play some role, however major or minor, in the grand story of our destinies turning out for the better. This is why we are constantly plagued with feelings of inadequacy. "I am not good enough!" And insufficiency: "I am not doing enough." It' s why we have such a hard time accepting the free git of salvation offered by Jesus as a truly free gift. We want to earn it... This warped desire lies at the bottom of every sin we do...

We sin because our affections for the Gospel are weak, while our affections for our own provision are strong. We need the Gospel to rewire our hearts.  

We all take the good desire God put in our hearts to enjoy Him and obey and twist it into a works-based economy built to purchase our own salvation...

And it is into this predicament that Jesus enters... 

Jesus lived perfectly under God's covenant. He succeeded where Adam and Eve failed. Therefore His reward is the original reward offered in the garden—an eternally blessed life with God. 

However Jesus did not claim this reward for Himself. Instead He took the punishment of the covenant, that we earned. He took the full weight of death... death that we earned. He also gave us the blessing of life He earned... He credited to us. 

Now there is a new covenant extended to all humanity. This is the covenant of grace. You can't work for this covenant. You can only have faith in its work...

All the salvation for which you long has been earned for you in Jesus. Therefore we can look to Him and rest. We have been saved by the good works of Jesus...

We can do nothing to earn it or deserve it...

And rehearsing this story to your heart is how you grow affections that rewire your heart away from sin and toward God...

We sin when we lose faith in the Gospel. Which means we can only begin to kill sin when we fortify ourselves inChrist. Only then can the Gospel rewire our hearts.